1. Field of the Invention
The present invention relates to a semiconductor device having a circuit composed of a thin film transistor (hereinafter referred to as TFT). For example, the invention relates to an electro-optical device represented by a liquid crystal display panel and to electronic equipment mounted with the electro-optical device as a component.
In this specification, a ‘semiconductor device’ refers to a device in general that utilizes semiconductor characteristics to function, and electro-optical devices, semiconductor circuits, and electronic equipment are semiconductor devices.
2. Description of Related Art
A thin film transistor (hereinafter referred to as TFT) can be formed on a transparent glass substrate, and hence its application to an active matrix liquid crystal display (hereinafter referred to as AM-LCD) has been developed actively. A TFT utilizing a crystalline semiconductor film (typically, a polysilicon film) can provide high mobility, making it possible to integrate functional circuits on the same substrate for high definition image display.
An active matrix liquid crystal display device requires million TFTs for pixels alone when the screen is to have high definition. Its functional circuits also need TFTs to further increase the number of required TFTs. Each of these TFTs has to have secured reliability and operate stably in order to realize stable operation of the liquid crystal display device.
However, the TFT is considered as not so equal in terms of reliability to a MOSFET that is formed on a single crystal semiconductor substrate. The TFT experiences lowering of mobility and ON current when it is operated for a long period of time, as the MOSFET suffers from the same phenomena. One of causes of the phenomena is characteristic degradation due to hot carriers that accompany enlargement of a channel electric field.
The MOSFET, on the other hand, has the LDD (lightly doped drain) structure as a well-known reliability improving technique. This structure adds a low concentration impurity region inside a source/drain region. The low concentration impurity region is called an LDD region. Some TFTs employ the LDD structure.
Another known structure for the MOSFET is to make the LDD region somewhat overlap a gate electrode through a gate insulating film. This structure can be obtained in several different modes. For example, structures called GOLD (Gate-drain overlapped LDD) and LATID (Large-tilt-angle-implanted drain) are known. The hot carrier withstandingness can be enhanced by these structures.
There have been attempts to apply these structures for MOSFETs to TFTs. However, application of the GOLD structure (in this specification, a structure having an LDD region to which a gate voltage is applied is called a GOLD structure whereas a structure having merely an LDD region to which a gate voltage is not applied is called an LDD structure) to a TFT has a problem of OFF current (current flowing when the TFT is in an OFF state) being larger than in the LDD structure. For that reason, the GOLD structure is not suitable for a circuit in which OFF current should be as small as possible, such as a pixel matrix circuit of an AM-LCD.